The present invention relates to a process for pre-expanding thermoplastic resin particles, and more particularly to a process for pre-expanding thermoplastic resin particles capable of giving molded articles having small shrinkage and excellent surface appearance.
As to a process for pre-expanding thermoplastic resin particles, it is described in Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 197027/1983 that a volatile blowing agent is impregnated into thermoplastic resin particles dispersed into water under pressure by heating in a pressure vessel and the mixture of the thermoplastic resin particles and water is released into a low pressure zone through one orifice plate under a vapor pressure of the volatile blowing agent to preexpand the thermoplastic resin particles. However, the pre-expanded particles obtained by the method described in Tokkyo Kokai No. 197027/1983 are fine in cell diameter. When the expansion molding is carried out by using the pre-expanded particles having fine cell diameters, the obtained molded articles are large in shrinkage. Also, when the pre-expansion is carried out by using thermoplastic resin particles having small diameters, there is a tendency that the obtained pre-expanded particles are small in cell diameter and therefore the articles obtained by molding the pre-expanded particles are large in shrinkage.
It is an object of the present invention to provide a process for pre-expanding thermoplastic resin particles, which can improve qualities of the molded articles by making their cell diameter large.
The above and other objects of the present invention will become apparent from the description hereinafter.